The maximum SMS body length is actually 140 bytes, which equates to 160 GSM-encoded characters (7 bits each) or 70 unicode-encoded characters (2 bytes each).
If you are seeing a maximum message length of only 70 characters this is an indication that you are requesting unicode encoding. If you are using an application or script to submit your requests it may be that your program code is determining you need to use unicode when this is not the case.
The majority of accented French characters are included in the gsm character set. If you are sending in a Western European language we would recommend forcing the message body type to text (i.e. not including the &type=unicode in your API request).
If you are using the CM direct web application for sending SMS messages you will be automatically informed wether your using the standaard GSM-encoded characters or Unicode.